In recent years, use of mobile communications devices for voice telephone services, email or text messaging services and even multi-media service has become commonplace, among mobile professionals and throughout the more general consumer population. Mobile service provided through public cellular or PCS (personal communication service) type networks, particularly for voice telephone service, has become virtually ubiquitous across much of the world. In the USA, for example, competing public networks today provide mobile communications services covering most if not all of the geographic area of the country.
In addition to public networks, similar wireless technologies have provided relatively small scale networks for enterprise applications, typically offering wireless service analogous to private branch exchange (PBX) type service. Such a smaller scale private system includes several base stations, similar to but scaled down from those used in the public networks, in combination with a private circuit switch or more recently an Internet Protocol (IP) router or network, for providing communications between devices and with external networks. U.S. Pat. No. 6,970,719 to McConnell et al. and US application publication no. 2005/0059390 to Sayers et al. disclose examples of such private wireless cell phone networks. Although sometimes referred to as an “indoor cellular network” or “indoor system,” such enterprise systems need not be literally indoors and for example may offer coverage across an entire campus area. Alternatively, such an enterprise cellular communication system may be referred to as a “picocell” system, with the outdoor public cellular communication system covering a wider area being referred to as a “macro-cell” system.
As broadband IP connectivity to homes and offices has become more common, and the speeds of packet-switched communications equipment and the speed of processors have increased, a variety of applications have emerged that utilize IP packet transport as an alternative bearer for voice communications. Generally, such applications are referred to as voice-over packet services, however, the common forms based on Internet Protocol (IP) are referred to as “Voice over IP” or “VoIP” services. Although originally developed for wireline network transport through the Internet and through wireline intranets, VoIP services are now migrating to the wireless domain. Picocell systems, which use IP routing or frame switching for IP transport, utilize VoIP technology to support the voice services.
As an extension of these developments/deployments into the customer premises, particularly for residential or small business applications, equipment manufacturers have recently begun offering “femtocell” devices, e.g. for home installation. A “femtocell” system is a base transceiver system (BTS) forming a compact base station. In most recent examples, such compact base stations are equipped with VoIP capability and an IP interface, for example, for connection to a Fiber Optic Service (FiOS) modem, to a digital subscriber line (DSL) modem or to a cable modem. One such unit in a home or small business, for example, would allow mobile station users in the premises to make and receive calls via the existing broadband wireline service from the customer's Internet Service Provider (ISP), without consuming air-time minutes for wireless service that otherwise would use the public network of their mobile/wireless service provider.
It has been suggested that the deployment of femtocells will be particularly advantageous to a service provider as a way to improve service of the service provider's macro network in customer premises locations where the macro network service is less than optimum. For example, if a mobile station user may have weak coverage at his or her residence, installation of femtocell type BTS in the home effectively extends macro cell mobile network coverage into the home in a manner that substantially improves the customer's experience using the service provider's network.
At present, plans therefore are for the public mobile service provider(s) to distribute (sell or lease) the femtocell equipment to their public network customers. The femtocell is intended to work with a regular mobile station. To the mobile station, the BTS of such a femtocell appears like a normal base station of the public network. Deployment of femtocells, particularly in large numbers of customer premises, requires coordination with networks of the applicable public mobile service providers, which raises issues for such a service provider. One such problem relates to frequencies of femto operation, particularly in a manner that allows mobile stations to easily find and access the femtocell.
For example, femtocells may operate on frequencies other than those that the mobile service provider uses in the surrounding macro network region. Hence, the femtocell will need to direct mobile devices of the service provider's customers onto the traffic bearing carrier on a frequency band of femtocell operation. One approach employs a “beacon” signal on a pilot channel to serve as a trigger to the mobile station. Essentially, the femtocell broadcasts the pilot on a macro network frequency carrier; and the beacon on the pilot channel directs a mobile station that attempts to acquire service through the femtocell over to the frequency that the femtocell uses for its traffic functions.
Hence, for a public mobile service provider deployment, femtocells that the service provider provides to its customers are configured to operate such beacons on the provider's own licensed frequencies. In a heavily populated area, a major mobile service provider will have licensed substantial portions of the spectrum for its uses. To allow mobile stations to find, lock-on and register through network base stations, the network base stations broadcast in the region may operate on any number of channels. In some areas, base stations of a major service provider such as Verizon Wireless may broadcast pilot channels on ten or more different frequencies. In operation, each mobile station runs an algorithm to help it quickly find a preferred one of the service provider's pilot channels, for example, by selecting a pilot frequency from among the service provider's pilot frequencies based on a hash of the mobile identification number (MIN) that the provider has assigned to the mobile station. Numerous mobile stations seeking access through femtocells therefore may be looking for a relatively large number of different preferred pilot channel frequencies.
Although a mobile station may be able to search for other channel frequencies, in the event that it fails to detect the preferred channel frequency, the requisite searching may take a relatively long time during which the mobile station is without network service. To facilitate fast access to a femtocell, it has been suggested that each femtocell could provide a means to broadcast on all of the service provider's pilot channels, so as to allow mobile stations to find the femtocells. For example, in a network with spectrum to support ten carriers, present technology describes a process in which the femtocell generates a pilot beacon on each of the ten carriers, in sequence, based on provisioning the femtocell to identify the appropriate carriers. These pilot beacon carriers are utilized to direct an approaching mobile device towards a dedicated frequency channel and carrier for localized use in the coverage area of the particular femtocell. In operation, the femtocell transmits a pilot beacon on one carrier for a short period of time, then transitions to the next carrier, and so on, in such a manner that the femtocell cycles through all ten carriers repeatedly. Stated another way, the femtocell sequentially “hops” through the appropriate macro-network carrier frequencies for its pilot beacon transmissions. Carrier generation, as well as the time for transmission on each frequency and the time to cycle through all carrier frequencies for re-transmission of a given pilot channel carrier frequency, will vary depending upon the number of carriers that the femtocell is required to support in any given area and/or for a particular service provider's customers.
Depending upon the spectrum owned by a network provider, the number of macro-network service providers present in the public network can be quite large. Hence, it is expected that the number of beacons a femtocell would be expected to generate would match the macro-network carrier list, and as such be equally large. Cycling through so many carriers, in the designated frequency hopping sequence, is inefficient. The requirement to generate, transmit and cyclically regenerate a large number of carriers, induces a system acquisition time lag component into the ability of an approaching mobile device to “find” the femtocell and “lock on” to it for service. For example, the femtocell broadcasts the pilot beacon on any one carrier only for a small percentage of the time, approximately 10% in our ten carrier example. A mobile station entering the coverage area of the femtocell may not detect its preferred pilot frequency for a considerable period, while the femtocell is cycling through the other carriers. Also, if there is some interference during the first broadcast on the preferred carrier, the mobile station may have to wait through one or more additional long cycles until adequate reception on the appropriate pilot carrier frequency. During the time between leaving the macro-network service and “finding” the femtocell, the mobile device would indicate “no-service.”
Hence a need exists for a technique to improve or optimize pilot transmission time and cycle for re-transmission time if needed for pilot channels for idle state acquisition of femtocells by mobile stations.